Data storage systems can fall into a plurality of categories, such as Network Attached Storage (NAS) and Storage Area Networks (SAN). A NAS system can be a stand-alone, network-accessible, storage device that can provide file-level access to electronic data. A SAN array can be a dedicated storage system that can connect numerous storage resources to one or many hosts. A SAN can provide block-level access to electronic data through one or more protocols, e.g., (SCSI-based protocols such as Fiber Channel or iSCSI), which can be used by a connected host to provide a file system.
Storage systems can be employed that include two storage processors and one or more storage devices. Examples of such storage devices include disk storage arrays and network storage arrays. From the perspective of a host that issues IO requests to the storage processors to read, write, copy, and/or allocate data storage on the storage devices, the storage devices appear accessible from either storage processor. In some storage systems, in order to upgrade the storage system both storage processors are replaced (e.g., with storage processors with increased memory and/or increased processing power). In some storage systems, in order to perform the upgrade both storage processors are shut down at the same time and replaced. When both storage systems are shut down, the storage system cannot process IO requests from hosts.